It is a 308 case necked up to 35 caliber. I think they added a rim to the case. It came out in Winchesters Big Bore along with the same case called the 307 Winchester. They also chambered it in the 375 Winchester. The sales never came about and I think only the 307 remains if even it does. I have a hundred or so new brass for the 356 if anyone is interested. If I recall they used a 358 caliber bullet which shoul still be available. I think Marlin chambered some of these and I know they chambered the 375 Winchester. It was an attept to breath some new life into the Winchester lever guns. For short range woods hunting they were a good gun but suffered from the same problem all tubular magazine guns face. The inability to handle pointed bullets. Sorry I don't recall an awful lot about them I had a 375 and a 307. I had a 356 ordered thus the brass that I have. I changed my mind and never bought the 356. There will be more knowledgeable people who can tell you more.